Among surface light-emitting bodies, an organic electroluminescent (EL) light-emitting element is expected to be used in a flat panel display and also in a new generation lighting device that is used in place of a fluorescent bulb and the like.
The structure of the organic EL light-emitting element is diversified, that is, from a simple structure in which an organic thin film to be a light emitting layer is merely interposed between two electrodes to a structure in which a light emitting layer is provided and an organic thin film is multi-layered. As an example of the latter multi-layered structure, those obtained by laminating a hole transporting layer, a light emitting layer, an electron transporting layer, and a negative electrode on a positive electrode formed on a glass substrate are exemplified. The layer interposed between the positive electrode and the negative electrode entirely consists of an organic thin film, and the thickness of each organic thin film is extremely thin, that is, only several tens of nm.
The organic EL light-emitting element is a laminate of thin films, and based on a difference in refractive index between materials of each thin film, the total reflection angle of the light between the thin films is determined. Under the current circumstances, about 80% of the light generated from the light emitting layer is trapped inside the organic EL light-emitting element and cannot be extracted to the outside. Specifically, when the refractive index of the glass substrate is 1.5 and the refractive index of an air layer is 1.0, a critical angle θc is 41.8° and the light with the incidence angle lower than the critical angle θc is emitted from the glass substrate to the air layer. However, the light with the incidence angle higher than the critical angle θc undergoes total reflection and is trapped inside the glass substrate. For such reasons, it has been desired to extract the light trapped inside the glass substrate on the surface of the organic EL light-emitting element to the outside of the glass substrate, that is, to improve light extraction efficiency.
Furthermore, regarding an organic EL light-emitting element which performs isotropic light emission, it is desired to have not only improvement in the light extraction efficiency but also a decrease in the emission angle dependence of the wavelength of light emitted from the organic EL light-emitting element. That is, it is desired to have a small difference in emission angle according to the wavelength when light emitted from the light emitting layer passes through the glass substrate to result in light emission from the glass substrate. In other words, the wavelength dependence of the light distribution emitted from the glass substrate is desired to be as small as possible.
In order to solve the problems described above, Patent Document 1 proposes a lens sheet including a diffusion layer in which particles increases toward a light emitting surface side and a recessed and projected structure layer including particles.